1. Field of the Disclosure
The present disclosure relates to a vertical external cavity surface emitting laser (VECSEL) device, and more in particular, to a VECSEL device with an improved structure to improve the gain of each quantum well layer in a multiple quantum well layer having a periodic gain structure.
2. Description of the Related Art
A vertical cavity surface emitting laser (VCSEL) radiates a single longitudinal beam of light having a very narrow spectrum. The projection angle of the beam is small and thus the coupling efficiency is high. Also, since monolithic integration with other appliances is easy due to the characteristics of the surface light emission, the VCSEL is appropriate as a pumping light source.
However, a conventional VCSEL cannot radiate a single lateral light beam as easily as an edge emitting laser diode (LD). Also, the radiation area should be 10 μm or less for a common single lateral mode operation, and even then the VCSEL enters a multiple mode due to a thermal lens effect as the light output increases. Thus, the maximum power of the output single lateral light is 5 mW.
A vertical external cavity surface emitting laser (VECSEL) provides the advantages of the VCSEL described above and simultaneously achieves a high power operation. The VECSEL has an external mirror in place of the upper DBR of the VCSEL, and thus has an increased gain area and can obtain an output power of 100 mW or more. Recently, a VECSEL device having a periodic gain structure with quantum wells (QW) arranged periodically has been developed to compensate for the disadvantages of surface emitting lasers (SEL) which cannot obtain as much gain as edge emitting lasers because of a lesser gain volume than edge emitting lasers. Also, a VECSEL device has been developed to uniformly pump a large area since electric pumping does not provide uniform carrier injection for a large area.
A strain compensation layer that compensates for strain in the quantum well layer to suppress structural defects in a VECSEL structure having a periodic gain has been suggested by Kuznetsov et al. [“Design and Characteristics of High-power Diode-Pumped Vertical-External-Cavity Surface-Emitting Semiconductor Lasers”, Kuznetsov et al. IEEE J. Selected topics in Quantum electronics, Vol. 5(3), p. 561, 1999].
However, in a VECSEL device, the strain compensation layers completely offset strains around the quantum well layers, and when all the strains are offset, the maximum gain of the quantum well layers is lowered. Therefore, a structural improvement of the VECSEL is required in the VECSEL device including the strain compensation layers to obtain the maximum gain of the quantum well layers.